In a previously known circular diffuser of the foregoing kind, one or more inner diffuser cones are provided which is or are axially adjustable relative to the outer diffuser cone by means of a screwthreaded spindle which is secured to the innermost cone at one end and at the other end engages rotatably in a central web secured to the outer diffuser cone. By rotating the innermost cone, which is in the form of a frusto-cone in one or the other direction the spindle screws into or out of the central web, whereby the relative axial distance between the outer cone and the inner cone or the assembly of inner cones is varied as desired. While such construction serves its purpose, it is rather slow and cumbersome in operation.